1. Field of the Invention
The present invention relates to a lock-up clutch of a fluid transmission device, which is mounted between a pump impeller and a turbine impeller and capable of connecting the pump impeller and the turbine impeller directly to each other.
2. Description of the Related Art
There is such a conventionally known lock-up clutch including a driving friction plate and a pair of follower friction plates sandwiching the driving friction plate, these driving and follower friction plates being disposed between a turbine impeller and a side cover connected to a pump impeller to cover the a back of the turbine impeller, wherein the lock-up clutch is brought into an engaged state by urging the friction plates all together against an inner wall of the side cover by a hydraulic pressure, as disclosed in Japanese Patent Application Laid-open No.7-4497.
The above known lock-up clutch has an advantage that a large torque capacity can be provided by using the plurality of friction plates. However, during connection, an urging force applied to all of the friction plates is received by the side cover and for this reason, it is necessary to form the side cover to have a high rigidity, for example, by increasing the thickness of the side cover. This is inconvenient for providing a reduction in weight of a fluid transmission device.
Accordingly, it is an object of the present invention to provide a lock-up clutch of a fluid transmission device, wherein a large torque capacity can be ensured, while enabling the connection without bearing the urging force applied to the friction plates by the side cover, thereby contributing to a reduction in thickness of the side cover and the like and in its turn, to a reduction in weight of the fluid transmission device.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a lock-up clutch of a fluid transmission device mounted between a pump impeller and a turbine impeller and capable of connecting the impellers directly to each other, comprising an annular clutch outer mounted in a side cover connected to the pump impeller to cover a back of the turbine impeller, a clutch inner disposed radially inside the clutch outer and connected to the turbine impeller, a driving friction plate axially movably connected to the clutch outer, first and second follower friction plates connected to the clutch inner for movement toward and away from each other with the driving friction plate interposed therebetween, an inner chamber defined between the first and second follower friction plates, and an outer chamber defined in the side cover and faced by outer sides of the first and second follower friction plates so that the inside of the inner chamber is put at a pressure higher than that in the outer chamber to bring the lock-up clutch into a disengaged state in which the follower friction plates are spaced apart from the driving friction plate, and the inside of the outer chamber is put at a pressure higher than that in the inner chamber to bring the lock-up clutch into an engaged state in which the follower friction plates are in pressure contact with the driving friction plate.
With the first feature, in the engaged state of the lock-up clutch, the pair of follower friction plates are brought into a pressure contact with the opposite sides of the driving friction plate by putting the inside of the outer chamber at the pressure higher than that in the inner chamber and hence, a large torque capacity can be provided. Moreover, any of the driving friction plate and the follower friction plates cannot be urged directly against the inner wall of the side cover and hence, the side cover need not be formed especially to have a high rigidity, and can be formed to have a small thickness to the utmost, thereby providing a reduction in weight in the fluid transmission device.
According to a second aspect and feature of the present invention, in addition to the first feature, the clutch outer formed separately from the side cover is secured to the side cover.
With the second feature, even when the clutch outer is formed to have a large thickness to increase the torque capacity thereof, the formation of the clutch outer can be carried out independent of the side cover, resulting in substantially no increase in weight of the torque converter T.
According to a third aspect and feature of the present invention, in addition to the second feature, the clutch outer is comprised of a plurality of sector members arranged circumferentially, each of the sector members being secured to the side cover.
With the third feature, the plurality of sector members forming the annular clutch outer can be fabricated at a good yield from a material such as a steel plate, whereby the clutch outer can be produced at a low cost.
According to a fourth aspect and feature of the present invention, in addition to any of the first to third features, the clutch outer comprises a pair of sidewalls axially opposed to each other, and a peripheral wall integrally connecting the sidewalls to each other on the side of an outer periphery, the clutch outer having a plurality of narrower portions and a plurality of wider portions, which are formed alternately thereon, the narrower portions having a smaller distance between the sidewalls, and the wider portions having a larger distance between the sidewalls, and transmitting claws projectingly provided on an outer periphery of the driving friction plate are axially movably disposed within the narrower portions, and damper springs are accommodated within the wider portions with their opposite ends resiliently engaged with end faces of the narrower portions and the transmitting claws.
With the fourth feature, the ends of the damper springs can be supported with a good stability by utilizing stepped portions between the narrower and wider portions, and the buffering transmission of the torque between the springs and the transmitting claws can be conducted reliably.
Yet further, according to a fifth aspect and feature of the present invention, in addition to any of the first to third features, the clutch inner is releasably engaged with the turbine impeller.
With the fifth feature, in assembling of the fluid transmission device, the lock-up clutch can be assembled to the side cover independent of the turbine impeller, and after assembling of the lock-up clutch, the turbine impeller need only be accommodated within the side cover, whereby it can be connected easily to the clutch inner, leading to an extremely good assemblability.
The above and other objects, features and advantages of the invention will become apparent from the following detailed description of the preferred embodiment with reference to the accompanying drawings.